Chlamydia trachomatis infection in urogenital tract, if untreated, can cause severe complications. Due to lack of obvious symptoms, many C. trachomatis-infected individuals are undiagnosed and untreated. Thus, there is an urgent need for an effective anti-C. trachomatis vaccine. Currently, no licensed C. trachomatis vaccine is available, mainly due to lack of knowledge on protective antigens in C. trachomatis. The failure of human trachoma vaccine trials using formalin-fixed whole C. trachomatis organisms as vaccines >50 years ago suggests that alternative approaches are necessary for developing a safe and effective C. trachomatis vaccine. It is now known that a Th1-dominant and IFNg- dependent T cell immunity is required for controlling C. trachomatis infection, suggesting that an effective vaccine should include T cell antigens that can induce Th1-dominant responses. However, past extensive efforts in identifying chlamydial T cell antigens have so far yielded no effective vaccines. We have now expressed all 908 C. trachomatis open reading frames (ORFs) and successfully used them to map B cell antigens in C. trachomatis-infected animals and humans. However, attempts to map T cell antigens at the whole genome scale have been unsuccessful due to lack of sensitive assay systems for simultaneously screening all C. trachomatis ORFs for T cell recognition. To overcome these difficulties, we have developed a highly sensitive antigen presentation assay that has allowed us to identify 5 novel T cell antigens after screening 90 ORFs. One of antigens, CT858 (CPAF), has been shown to induce partial protection against C. muridarum infection in mice. Thus, we hypothesize that T cell antigens identified using the whole genome scale screening assay with T cells from hosts that have acquired protective immunity can induce protective immunity against C. trachomatis infection. To test the hypothesis, we are proposing to identify all potential T cell antigens encoded in C. trachomatis genome and plasmid by utilizing the newly established sensitive T cell assay and capitalizing on the already available 908 fusion proteins and to further evaluate their protection efficacy in a C. trachomatis induction of mouse infertility model. The proposed study, representing the first ever truly whole genome scale screening for T cell antigens followed by evaluation in a C. trachomatis infection-induced infertility of human HLA-DR4 transgenic mouse model, will significantly advance Chlamydia vaccine development.